The flue gases discharged from the melting vessel must be cooled and purified before being discharged into the atmosphere according to the specifications of TLA-Luft (Technical Instructions for Clean Air Maintenance) or other national regulations for clean air maintenance in, e.g., steel-making, especially electric steel production in arc furnaces or induction furnaces. Usually a one-step dry purification is carried out, e.g., in a cloth filter plant preceded by a water-cooled gas cooling section, in order to reduce the dust content in the purified waste gas to, e.g., 10 mg/Nm.sup.3.
Flue gases containing dioxins and other toxic substances are inevitably released when scrap metal is charged into the melting vessel.
Various comparative studies and measures for reducing the dioxin emissions from waste incinerator plants, especially those used for domestic waste, are described in the VDI-Berichte Nos. 634, 1987, pp. 515-539.
According to these investigations, dioxins (PCDDs) and furans (PCDFs) are formed due to the incineration of PCVs as well as PCBs, chlorophenols and chlorobenzenes, which are present in small amounts in waste, e.g., as dielectrics, wood preservatives, disinfectants, preservatives, etc., and are again found on the dusts present in the flue gases and consequently in the filter dust after cooling.
Investigations on flue gas purification plants of waste incinerator plants, which operated according to the so-called dry process or the quasi-dry process, have demonstrated filtration efficiencies exceeding 99.0% for PCDDs and PCDFs.
In the dry process, flue gases with a temperature of 255.degree.-270.degree. C. are quenched to 110.degree.-140.degree. C. with water, and mixed with dry Ca(OH).sub.2 in a separate reactor.
In the quasi-dry process, the flue gas is quenched with milk of lime. The metering of milk of lime is regulated as a function of the flue gas conditions such that the liquid will completely evaporate, and the suspended calcium hydroxide will be present in the flue gas in the particulate form. The solids are then separated by cloth filters in both processes.
DE 40 34 417 A1 describes the preparation and the use of highly reactive dry reagents for waste gas and waste water purification which contain, e.g., mixtures of hydrated lime or calcium hydroxide with additives that are characterized by high purification performance and the possibility of safe disposal of the reaction products on landfills.
These reagents generally have high porosity and a correspondingly large specific surface. The reagents are used for purifying waste gases and flue gases predominantly in a wet, quasi-dry, or dry process, as well as directly in the waste gas flow in front of an electrostatic precipitator of a flue gas purification plant.
DE 38 27 086 A1 discloses a process and a device for the thermal decontamination of dioxin- and furan-containing filter dusts, which also contain heavy metal compounds at the same time.
The material to be decontaminated is melted down at a temperature between 1,400.degree. C. and 1,600.degree. C. and for a predetermined corresponding hold time in an inductively heated crucible furnace, and is then brought to the liquid state of aggregation, while the dioxins and furans are destroyed and heavy metals are bound in the melt. The flue gas generated in the crucible furnace is passed through a high-temperature section, in which complete inertization or decontamination of the flue gas takes place at high temperatures (at least 1,200.degree. C.) and hold times exceeding 2 sec.